The present invention relates generally to optical filters, and more particularly to an optical filter having variable spectral transmittance function of selectable shape.
In the calibration and use of certain optical instruments, such as a photometer, colorimeter, spectroradiometer or spectrophotometer, a specific spectral transmittance function (STF) is required. For example, the spectral sensitivity of a photopic photometer must closely match the CIE 1924 photopic luminosity function, which requires passing incoming light through a filter having an STF tailored to the spectral sensitivity of the photodetector of the photometer, the STF of the optical path, and the CIE luminosity function. Simple single-layer absorption filters can be used for this purpose but often yield poor accuracy because of deviations from the ideal STF at various wavelengths.
Accuracy can be improved by combining filters so that the composite filter more closely matches the ideal STF, but selection of the correct set of filters for a given photodetector and optical path STF is laborious; in a layered filter, photometer sensitivity suffers because light is lost in each layer; for a mosaic filter, the correct filter material and size for each piece of mosaic must be determined; aging or contamination may cause changes in spectral sensitivity of the photodetector or in the STF of the optical path or the filter; and component replacement in the photodetector or associated optical system may require a new filter to match unique spectral characteristics of the component.
A fixed-template filter defining a spectrum forming element and a spatial aperture with shape required to produce a desired STF can yield both high accuracy and sensitivity, but construction is an iterative, time consuming process which results in a device not easily adjustable to changes in spectral sensitivity.
In fixed-template filter colorimeters, each of three templates must be positioned in turn with respect to the spectrum forming element so that the corresponding tristimulus value can be measured, which means that the three tristimulus values cannot be measured simultaneously and a highly accurate positioning mechanism must be provided. Beamsplitters may be used to illuminate the three filters simultaneously,but this reduces sensitivity and yields a bulky device.
In certain spectroradiometers and spectrophotometers an STF is required to measure the spectral distribution of light, and is typically achieved using either a grating monochromator or prism monochromator to disperse light into a spectrum, portions of which are selected by a mechanically movable slit the measurements from which may be subject to positioning errors because of mechanical imperfections.
Finally, special STFs can be desired when designing light sources for light-modulating displays. Often the spectral selection characteristics of these displays are less than ideal because materials are not available which have both the desired spatial and spectral light modulating characteristics, particularly for color displays.
The present invention substantially solves or reduces in critical importance problems with existing systems by providing a general purpose light filter having an STF which can be altered rapidly and easily under computer control and can assume substantially any shape, but having no moving mechanical parts. Practical uses involve the visible wavelengths, however, the potential spectral range is theoretically much broader.
The invention has substantial utility for systems which do not require passing an image containing spatial modulation but which do require passing light (which need not be spatially homogeneous) through an STF which is not readily obtained with ordinary optical filter. These systems may include photometers, colorimeters and devices using monochromators, such as spectroradiometers and spectrophotometers, and light sources for light modulating monochrome or color displays for which an ideal illuminating spectral radiance distribution is difficult to achieve.
It is therefore a principal object of the invention to provide an improved optical filter.
It is a further object of the invention to provide an optical filter having variable spectral transmittance function.
These and other objects of the invention will become apparent as the detailed description of representative embodiments proceeds.